When you fire up an espresso machine, it’s the hot water and steam that ultimately convert ground coffee into a beverage. These two ingredients are the fuel that espresso machines run on.
Now, to generate power from fuel, engines typically require an ignition chamber. This in the case of coffee machines is known as the boiler and it comes in three different forms.
- Single Boiler
- Double Boiler
- Heat Exchanger (HX)
The Single Boiler happens to be the cheapest, as it relies on just one element for both water heating and steam generation. This build makes it impossible to brew your espresso and froth milk concurrently.
If you want a cappuccino or latte, for instance, you’ll have to wait out the two phases – water is heated to make espresso, and then steaming follows to froth the milk. This sequencing, unfortunately, increases the risk of temperature inconsistencies between the espresso and the milk.
So, of course, an espresso machine with such a sequenced boiler might not be very efficient for cappuccinos. But then again, on the brighter side, it would be a decent budget option for someone who enjoys black coffee.
The opposite of that is the Double Boiler, which comes with two separate heaters that operate independently. This is essentially what you’ll find in premium professional espresso machines, as the double capacity supports simultaneous brewing and milk frothing, which translates to stable beverage temperatures.
But, since double boiler systems are comparatively pricey, you could alternatively go for a machine with a Heat Exchanger.
This category makes a fair compromise between Single and Double Boilers by running the heaters semi-independently. That means that although the machines may not be as powerful as Double Boilers, they are guaranteed to be much more stable than Single Boilers.